The present invention concerns a process for the separating mixtures of substances by means of centrifugal forces, particularly for the filtering and/or sedimentation of solids from suspension. The mixture of substances is centrifuged in a cake-forming chamber until a cake of solid particles is formed. The cake may be washed by flooding. A particular feature of the present invention is that, in forming the cake, sedimentation and filtration are effected at the same time and in the same place. The invention also concerns an apparatus for the realization of such a process.
The use of centrifugal forces for the separation of mixtures of substances is well known, particularly in chemistry, pharmacology and processing technologies. The centrifuges used in these processes may be divided fundamentally into installations operating continuously and whose operating discontinuously. In centrifuges operated discontinuously, individual batches of the mixture substances are filled into the centrifuge and processed in succession. In continuously operating centrifuges, the mixture flows continuously and permanently separated material is obtained.
An example of a continuously operating centrifuge is the so-called pusher centrifuge. In this centrifuge, the filter chamber is not closed in the axial direction, and a pusher axially displaces the material in short intervals. The material is thus arranged in varying concentrations along the axis of the centrifuge.
An example of a discontinuous centrifuge is the so-called stripping centrifuge. In this centrifuge, a particular amount of the mixture to be separated is filled into a filter drum at a time. The drum is then rotated until the solid and liquid components of the material are completely separated. The solid cake remaining is stripped from the drum.
Both known centrifuges, the stripper and the pusher centrifuges, have different disadvantages with respect to the requirements arising in the separation of substances.
Pusher centrifuges operate with a filtering chamber that is not closed in the axial direction, so that material mixtures with low solids concentrations cannot be processed in them. If materials with low solids concentrations were used, there would be a danger of overflooding the layers. Furthermore, pusher centrifuges cannot produce filtrates entirely free of solids because no straining clothes can be used. Another disadvantage of pusher centrifuges comprises that they do not permit good separation of mother and wash liquids. In certain cases, the grains (solid particles) may be excessively stressed in pusher centrifuges.
The main disadvantage of stripping centrifuges (with or without siphons) is that the flow rate, and thus the separating capacity, is relatively low. As individual batches must be processed in succession, the rotating velocity of the centrifuge must be altered frequently. This is not only highly labor intensive, but also a waste of energy. Because of the high stripping forces required, the structural cost of such centrifuges is very high. There is also a danger of rattling vibrations. Finally, the processing of grains in stripping centrifuges is not entirely satisfactory, because the stripping blades treat the solid cake in an extremely rough manner.